The present invention relates to a re-chargeable secondary battery, and more particularly to a explosion-resistant re-chargeable secondary battery having an improved positive electrode structure.
In Japanese laid-open patent publication No. 5-74436, a winding-structured lithium ion secondary battery is disclosed, wherein positive and negative electrodes comprise metal foils coated with an active material and provided with metal tabs connected to electrode terminals and the positive and negative electrodes are separated by a separator which has a property of shut down to a short circuit current.
In Japanese utility model application No. 61-71369, a battery is disclosed, wherein a sharp end portion of a porous core body is covered with a polyethylene tetrafluoride film or a polyester based resin film or a glass fiber film in order to prevent the sharp end portion of the porous core body from penetrating through a separator.
In Japanese utility model application No. 61-180902, a battery is disclosed, wherein a sharp end portion of a porous core body is covered with a fluorine resin film or a glass fiber film in order to prevent the sharp end portion of the porous core body from penetrating through a separator.
In Japanese utility model application No. 60-88225, a battery is disclosed, wherein a sharp end portion of a punched metal body is covered with a fluorine resin film or a glass fiber film in order to prevent the sharp end portion of the punched metal body from penetrating through a separator.
In Japanese laid-open patent publication No. 9-298054, a lithium ion secondary battery is disclosed, wherein an active material coating a positive electrode comprises a lithium manganate having a spinel structure.
The above prior arts are directed to prevent the electrode from penetrating through the separator and from contacting the opposite electrode to form a short circuit.
The above conventional lithium ion secondary battery has sufficient safety under normal states. However, if the above conventional lithium ion secondary battery is over-charged and has a current discharge between the positive and negative electrode through the separator, a temperature of the porous separator is increased and softened and melted, whereby the pores of the porous separator are closed to suppress ions from transferring through the porous-closed separator. As a result, the current between the positive and negative electrodes is reduced whereby the temperature is not increased up to the critical temperature which may case ignition of the battery Namely, the current is shut down.
In recent years, however, the lithium ion secondary battery has been developed in capacity for applicability to a large current usage condition. As the capacity of the battery is increased, the heat generation amount and the heat generation rate are increased so that the temperature of the battery is rapidly increased which causes the separator to completely melt to form a hole in the separator without causing the separator to exhibit a fuse function to close the pores of the separator due to slight melting of the separator. The formation of the hole in the separator allows the discharge current between the positive and negative electrode, whereby a further rapid temperature increase is caused, whereby an ignition or an explosion may appear. So long as it is required for advanced lithium ion secondary battery to increase their capacity as much as possible, the above problems with rapid increase in temperature of the battery are remarkable and serious to the advanced lithium ion secondary battery.
In the above circumstances, it had been required to develop a novel large capacitive secondary battery having sufficient safety not only in normal states but also in abnormal states such as over-charged states.